Binary tree - not able to add values

Question

In the below program, I am trying to populate a BST with the values present in an array arr. The program seems to run in a loop for a long time and then I get a segmentation fault. Can somebody please explain what I am missing here?

struct node {
        int value;
        struct node *left;
        struct node *right;
};

void add(struct node **root, int i);
int arr[14] = {30, 50, 25, 32, 45, 55, 20, 27, 31, 43, 47, 52, 88};

int main(void)
{
        struct node *root = malloc(sizeof(struct node));
        root->value = 35;
        root->left = NULL;
        root->right = NULL;

        struct node *start = root;
        int i;

        for (i = 0; i < 13; i++) {
                add(&root, arr[i]);
                root = start;
        }

        printf("%d\n", root->value);
        printf("%p\n", root->right);
        printf("%p\n", root->left);
}

void add(struct node **root, int i)
{
        while ((*root) != NULL) {
                printf("left: %p\n", (*root)->left);
                if (i < (*root)->value) {
                        add(&((*root)->left), i);
                } else {
                        add(&((*root)->right), i);
                }
        }
        *root = malloc(sizeof(struct node));
        (*root)->value = i;
        (*root)->left = NULL;
        (*root)->right = NULL;
}

Answer 1

You're using recursion, so there is no reason for the while to even be there. It should be an if-else logical construct:

void add(struct node **root, int i)
{
    if (*root)
    {
        if (i < (*root)->value) {
            add(&((*root)->left), i);
        } else {
            add(&((*root)->right), i);
        }
    }
    else
    {
        *root = malloc(sizeof(struct node));
        (*root)->value = i;
        (*root)->left = NULL;
        (*root)->right = NULL;
    }
}

If the goal was to eliminate the recursion, then the solution is to use the while-loop, but move root down the tree until it addresses an empty node:

void add(struct node **root, int i)
{
    while(*root)
    {
        if (i < (*root)->value) {
            root = &(*root)->left;
        } else {
            root = &(*root)->right;
        }
    }
    *root = malloc(sizeof(struct node));
    (*root)->value = i;
    (*root)->left = NULL;
    (*root)->right = NULL;
}

Finally, the for-loop in main unnecessarily resets root repeatedly. Properly done that loop should just look like this (assuming the goal is to host only the values in the array):

int main(void)
{
    struct node *root = NULL;

    int i;
    for (i = 0; i < 13; i++)
        add(&root, arr[i]);

    printf("%d\n", root->value);
    printf("%p\n", root->right);
    printf("%p\n", root->left);
}